This invention relates to an image recording device employing a method of developing invisible electrostatic latent images into visible images by two-component developing agent comprising toner and carriers, in which developing conditions are controlled to obtain optimum image density.
In a developing device which develops the electrostatic latent image on a carrier using a two-component developing agent comprising toner and a carrier, development is greatly dependent upon the ratio of weights of toner and carrier (termed toner concentration Tc) in the two-component developing agent. For instance, in case the toner concentration of the developing agent is less than the optimum concentration, the developed image would become whiter. Contrarily, in case the toner concentration is greater than the optimum concentration, the developed image would become darker and fogged. Therefore, the image on the transfer material which is transferred from the carrier would not be desirable.
To always obtain desirable image densities, it is required to control the concentration of toner in the 2-component developing agent so it may remain in a preset right level during development. Conventionally, a density control method has been proposed to keep the concentration of toner in the 2-component developing agent at a preset level by controlling the amount of supply of toner. This control method controls to keep the concentration of toner in the 2-component developing agent at a preset level by detecting changes of events such as permeability or volumetric change of the 2-component developing agent, change in image density of the developed image, change in color of the 2-component developing agent comprising toner and carrier whose colors are different and by controlling the amount of supply of toner according to the detected toner concentration.
However, this conventional method cannot be stable for a long period because it cannot escape from false detections, deterioration of photosensitive surfaces with the lapse of time, etc.
One of solutions to such problems is introduced in "2-Component Developer Concentration Control Equipment" (Patent Publication No.57-136669). To keep the optimum developing effect of the recording device stable for a long period, this 2-component developer concentration control equipment is equipped with two or more liquid level sensors in the developer to detect the volume of the 2-component developing agent in the developer and means to optically detect the toner concentration of a developed or recorded reference image on the sensitive surface. The central processing unit changes the liquid levels according to the optically detected image density and thus controls the concentration of toner in the 2-component developing agent to obtain constant developing performance with less carrier supply. With this, the above purpose has been attained temporarily.
Generally, documents to be image-recorded have different photographic densities therein. For documents having low photographic densities such as substantially white documents including lines or character patterns are image-recorded and developed continuously, it sometimes happened that the resulting image density was less than the image density corresponding to the toner concentration although the toner concentration was constant. In other words, the relationship between the toner concentration and the image density is broken. We have known that such events are often found in color electrostatic recording devices that reproduces color images by using 2-component developing agent for each color.
After researching into the cause of such events thoroughly, we have reached the following conclusions: As the result of analysis of events found in both of the above examples, we found that this event is closely dependent upon the time of agitation during which the developing sleeve and the agitating means in the developer rotate continuously during the image forming process, that is, the time in which the 2-component developing agent stays in the toner hopper. In the former example of image-recording of substantially white documents, the quantity of toner consumed per unit time is less than that of toner consumed in the normal status and consequently toner stays longer in the toner hopper. In the latter example of color-image recording, the 2-component developing agent for each color consumes a different quantity of toner and consequently, times in which toners for colors stay in the toner hoppers are different.
Judging from the fact that said events will occur even when the toner concentration is constant, we can assume that the said events are not due to the Spent Toner status in which toner components attach around carriers and the quantity of toner charge is reduced. Contrarily as the events are dependent upon the time period in which toner stays in the toner hopper, we can assume that the toner can have more chance to be in contact with the carrier as the toner stays longer in the toner hopper, that the quantity of toner charge increases, and that the excessive toner charge will affect the image concentration. Then, we have researched the relationship between the quantity of toner charge and the image density.
FIG. 4 illustrates how the image density CD changes as the quantity of toner charge Q/m (.mu.C/g) changes in case the toner concentration of a 2-component developing agent is constant.
In FIG. 4, we know that the image density CD goes down as the quantity of toner charge Q/m increases. To be more concrete, when the quantity of toner charge Q/m is 10 (.mu.C/g), the image density is approximately 1.4 and when the quantity of toner charge Q/m is 30 (.mu.C/g), the image density is approximately 0.5.
Such a relationship between the image density CD and the quantity of toner charge Q/m can be explained from the view point of a force (hereinafter called a developing force) that the toner in the 2-component developing agent which is magnetically held by the developing sleeve receives in the developing area in which an electrostatic latent image is developed.
Let the developing force be designated as Ft, the developing force Ft is approximately expressed as shown below. EQU F.sub.t =q.sub.t .multidot.E-k(q.sub.t .multidot.q.sub.c /r.sup.2)-q.sub.t (V.sub.B /R)
(where qt is the quantity of toner charge. E is the electric field strength made by an electrostatic latent image (hereinafter called a latent electric field). qc is the quantity of carrier charge. r is the distance between toner and carrier. VB is a voltage (hereinafter called a developing bias) applied to the developing sleeve. R is the distance between the surface of the photosensitive drum and the surface of the developing sleeve.)
In Equation 1, the developing force Ft is obtained by subtracting the force of attraction between toner and carrier (hereinafter called a Coulomb force) and a developing bias force acting on the toner from a force caused by the latent electric field E. Note that the force determining the developing force Ft is proportional to the quantity of toner charge qt (Q/m) but the Coulomb force that reduces the developing force Ft is apt to increase greater than the other forces as the quantity of carrier charge qc also increases as the quantity of toner charge qt (Q/m) increases. Therefore, the increase of the quantity of toner charge qt (Q/m) causes the developing force Ft to reduce and the reduction of the quantity of toner charge qt (Q/m) causes the developing force Ft to increase.
FIG. 5 illustrates how the image density CD changes according to the concentration (wt %: weight percentage) of toner in the 2-component developing agent stored in the developer.
In this figure, the 2-component developing agent comprises insulating magnetic carrier whose mean grain diameter is 20 .mu.m to 100 .mu.m (resin-coated; magnetic grains dispersed in resin) and toner whose mean grain diameter is 5 .mu.m to 15 .mu.m.
In FIG. 5, the solid line A indicates a CD-Tc characteristic curve in case the developing bias VB is 200 volts. In this case, the image density CD is about 0.4 for toner concentration of about 1 wt %, about 1.0 for toner concentration of about 5 wt %, and fixed to about 1.2 for toner concentration of 5 wt % or more. On the characteristic curve A, the image density CD is in the range of about 0.4 to about 1.2 with a wider contrast range. In this status, toner is well attached to the surface of the photosensitive drum which is the carrier of a latent image and unwanted image deviation will rarely be found. Therefore it is recommended to control the image density along the characteristic curve A.
The dash-dot line B shows the relationship between the toner concentration Tc (wt %) and the image density CD in case the time of agitating the 2-component developing agent becomes longer and the quantity of toner charge qt (Q/m) becomes much greater than that of solid line A. The other conditions such as the developing bias VB for the dash-dot line B are the same as those of the solid line A. In this case, as shown in Equation 1, as the Coulomb force increases greater than the other electrostatic forces, the force that attracts toner to the surface of the photosensitive drum reduces in inverse proportion to the Coulomb force and the image density goes down. To be more concrete, the dash-dot line B becomes fixed to about image density of 0.5 starting from toner concentration of 5 wt %. Therefore, we cannot obtain enough image density CD (in comparison with the original images) on the curve B and consequentially, the resulting contrast is not satisfactory, either.
The dotted line B represents the Tc-CD characteristic curve in case the quantity of toner charge falls under that of the solid line A because of an environmental change (e.g., increase of relative humidity). In this case, the force of attraction (Coulomb force) between toner and carrier is not enough and the image density CD becomes greater than that of the solid line A. To be more concrete, the image density CD reaches about 0.7 for toner concentration of 1 wt %, then fixed to about 1.4 for higher toner concentration. Accordingly, images developed along the dotted line C can have an enough contrast width. However, excessive toner attaches to the latent image and the developed image is far from the original one. What is worse, an image deviation is apt to occur.
This invention has been made to solve the above problems. The first purpose of this invention is to offer an image recording device that reproduces images of optimum image density by detecting any change (decrease or increase) of the image density while the toner concentration is kept constant and regulating the developing conditions.
FIG. 11a shows how the toner for each color is consumed according to the number of copies in a color image recording device that records color images by 2-component developing agents of several colors. FIG. 11b shows how the quantity of toner charge for each color changes according to the number of copies in a color image recording device. FIG. 11c shows how the optical image density CD changes according to the number of copies in a color image recording device.
The optical image density is the magnitude of light reflected from a standard toner image which the optical scanning system formed on the photosensitive drum according to an image signal corresponding to the light reflected by a standard density plate and which was developed under a preset electrostatic processing condition.
FIG. 11a is for an example of copying a certain colored document. The solid line BK represents how much the black toner is consumed as the number of copies increases. The standard consumption of black toner is about 80 mg each JIS-A4 size paper and about 240 g for 3000 sheets of JIS-A4 size paper.
The solid line Y represents how much the yellow toner is consumed as the number of copies increases. The standard consumption of yellow toner is about 50 mg each JIS-A4 size paper and about 150 g for 3000 sheets of JIS-A4 size paper. The solid line C represents how much the cyan toner is consumed as the number of copies increases. The standard consumption of cyan toner is about 50 mg each JIS-A4 size paper and about 150 g for 3000 sheets of JIS-A4 size paper. The solid line M represents how much the magenta toner is consumed as the number of copies increases. The standard consumption of magenta toner is about 10 mg each JIS-A4 size paper and about 30 g for 3000 sheets of JIS-A4 size paper. Namely, the amounts of toners in the developing agent to be consumed are different.
These characteristic lines assume that the developing characteristics of each color remain unchanged after 3000 copies.
For simplification of control, the developing sleeve and the agitating means in the developer of the conventional image recording device rotate continuously during the time of development in the image forming process. Therefore, the quantity of toner charge of each color in the developer varies since the toners stay longer in the developer. Some examples are explained below.
In FIG. 11b, the solid line BK represents how the quantity of charge of black toner (Q/m) changes as the number of copies increases. Namely, this line shows that the quantity of charge of black toner is approximately constant regardless of the number of copies. This is, we assume, because the toner concentration controller controls the supply of toner as it is consumed and consequentially there is little toner that stays long in the developer.
The solid line Y in FIG. 11b represents how the quantity of charge of yellow toner (Q/m) changes as the number of copies increases. The solid line C represents how the quantity of charge of cyan toner (Q/m) changes as the number of copies increases. The solid lines Y and C go up very slowly as the number of copies increases. We assume that this is because some excessive toner remains longer although the toner concentration controller controls the supply of toner as it is consumed.
The solid line M represents how the quantity of charge of magenta toner (Q/m) changes as the number of copies increases. The quantity of toner charge goes up quickly as the number of copies increases. We assume that this is because a lot of excessive toner remains longer although the toner concentration controller controls the supply of toner as it is consumed. As mentioned above, when a new developing agent for each color is loaded, the BK, C, Y, and M toners have approximately the same quantities of charges and have little influence on the developing characteristics. However, as copying advances, the developing agents are consumed differently and some kinds of toners will stay long in the developer, which causes a change in quantities of toner charges and a change in developing characteristics.
This difference in the quantities of toner charges in color developing agents causes the following events:
In FIG. 11c, the solid line BK shows how the optical image density CD of black toner changes as the number of copies increases. Namely, the optical image density (developing characteristics) of black toner is approximately constant independently of the number of copies. The solid line Y shows how the optical image density CD of yellow toner changes as the number of copies increases. The solid line C shows how the optical image density CD of cyan toner changes as the number of copies increases. These lines increase slowly as copying advances. Namely, the developing characteristics of yellow and cyan toners reduce as copying advances. The solid line M shows how the optical image density CD of magenta toner changes as the number of copies increases. The optical image density goes low extremely as copying advances. Namely, the developing characteristics of magenta toner reduces quickly.
As mentioned above, in an image recording device that continuously agitates developing agent in each color developer during the whole developing time, color developing agents are consumed differently and consequently the time periods during which color toners are agitated differ. This damages the developing characteristics, causing reduction in image density in monochromatic image reproduction, color unbalance in color image reproduction, etc.
This invention is made to solve the above-mentioned problems. The secondary purpose of the present invention is to offer a color image forming device that detects any change (decrease or increase) in the image density of color developing agents although the concentration of color toners are controlled at preset levels, regulates the electrostatic process conditions, and reproduces well-balanced color images of optimum image densities.